Plasticized polyvinyl butyral (PVB) sheet is used in the manufacture of laminate structures such as, for example: windshields for vehicles including automobiles, motorcycles, boats and airplanes; homes and buildings; shelving in cabinets and display cases; and other articles where structural strength is desirable in a glass sheet.
Conventional PVB sheet typically includes a plasticizer in order to increase the flexibility and processibility of the PVB sheet. Various plasticizers are conventional in the manufacture of PVB. Unfortunately, all plasticizers are not equally compatible with PVB. For example triethyleneglycol di-(2-ethyl hexanoate) (3GO), while offering some process advantages over other plasticizers, is not as compatible with PVB as, for example, dibutyl sebacate (DBS) or dihexyl adipate (DHA) or tetraethylene glycol di(2-heptanoate) (4G7). It is known, however, that the compatibility of a particular plasticizer in PVB can be increased by reducing the hydroxyl number of the PVB.
Manufacturers of PVB looking to improve the process of manufacturing PVB, and/or improve the performance of PVB in the various applications can face problems caused by changes to the composition of the polymer, or changes in the manufacturing process. In making such changes the performance of the product can be affected in ways that can result in unexpected problems.
In order to make product that is of consistent quality and useful in many of the applications described hereinabove, a PVB manufacturer typically monitors the performance of the product in various tests. For example, PVB should have acceptable tensile creep, impact resistance, haze, color, melt flow rate, and adhesion (for example, to glass). In making any changes to the composition of PVB or to the manufacturing process, the properties listed above should all remain acceptable for the product to be commercially acceptable and viable.
However, changing process parameters in a manufacturing facility is not always trouble-free. While evaluating the feasibility of making a change in the identity of the plasticizer used in their PVB manufacturing process, the Applicants encountered several problems. One problem encountered by the Applicants was that when changing the formulation of PVB by lowering the hydroxyl number in order to increase the compatibility of 3GO with PVB, an unexpected change in the tensile creep properties of the PVB was observed. Due to the nature of the change, the Applicants were not convinced that the PVB would be acceptable for commercial sale into applications wherein toughness and high impact resistance were important factors. While it was still desirable to lower the hydroxyl number of the PVB, the Applicants were faced with maintaining the above physical properties, particularly the tensile creep, of the PVB at a desirable level. Polyvinyl butyral is capable of stereoisomerism, due to the cyclic structures formed by the reaction of polyvinyl alcohol (PVA) with butyraldehyde. In EP 0402213 the ratio of the meso cyclic structure to the racemic cyclic structure in the PVB resin is related to the rigidity of the plasticized PVB sheeting. The recognition of the relationship of the stereoisomerism in PVB resin to the physical properties of plasticized PVB sheeting has not been fully appreciated in the patent literature. Further it has not been recognized or fully appreciated that critical physical properties of PVB sheeting can be manipulated by controlling the stereoisomerism of the PVB resin. Even further, a method for controlling the stereoisomerism of PVB resin during the manufacturing process has not been recognized or, at least, not fully appreciated in the art of PVB manufacture.
In the manufacture of PVB, it can be desirable to have a process for preparing plasticized PVB sheet whereby critical physical properties of the sheet can be controlled in spite of changing the manufacturing process for PVB, or the composition of the PVB resin used to make a PVB sheet, or a change in other components used to make the PVB sheet.